The present application relates to optical transceiver technology, for example, as used in a wireless optical network (WON).
Wireless optical networks (WONs) are becoming increasingly popular in the telecommunications market as a strategy to meet last-mile demand, enabling reliable high-bandwidth connectivity previous available only to customers directly connected to fiber or cable. An example of a WON is Airfiber""s OptiMesh system, which is described generally in U.S. Pat. No. 6,049,593, and co-pending U.S. patent application Ser. No. 09/181,043, entitled xe2x80x9cWireless Communication Network.xe2x80x9d
Historically used by military and aerospace industry, WON technology has evolved into systems with backup and redundant optical links, providing high reliability and fiber-like bandwidth to customers located up to a kilometer away from buried fiber. Such systems are being deployed to commercial buildings in urban area, breaking the so-called xe2x80x9clast-milexe2x80x9d bottleneck. These WONs provide higher bandwidth than Radio Frequency (RF) wireless systems and are considerably less expensive to deploy than laying fiber.
FIG. 1 illustrates an example of a WON application. As shown therein, facilities 104 (e.g., commercial office buildings) can be linked to a high bandwidth network 102 (e.g., a fiber-based network) by means of optical transceivers 106 and 107, which use xe2x80x9copen-airxe2x80x9d or xe2x80x9cfree-spacexe2x80x9d laser beams to maintain wireless, high bandwith communication links 108 among each other. The central, or main, optical transceiver 107 can have a communication link 110 (e.g., either wired or wireless) to the network 102, and thereby serve as a hub for the other optical transceivers 106.
FIG. 2 shows an example of a conventional wireless optical transceiver 106. As shown therein, the transceiver 106 is composed of two basic elements: an output channel 200 for transmitting a laser beam (modulated or otherwise impressed with data) to another transceiver in the WON, and input channel 202 for receiving a modulated laser beam from another transceiver in the WON. Each of the input and output channels is composed of three basic components. The output channel 200 includes a laser diode (LD), which emits a laser beam of a predetermined wavelength (in this example, 785 nanometers) that passes through a diffuser 206 and which is focused by optics 204 (e.g., a plano aspheric lens). An incoming beam, for example, from another transceiver in the WON, is received by optics 204 of the input channel 202, passed through a bandpass filter 210 and ultimately received by a photodetector (PD), e.g., an avalanche photodiode.
The open-air laser beams used by WONs to transmit and receive data pose a potential threat to human eye safety. The collimated, beam-like quality of a laser results in very high irradiance (also known as xe2x80x9cpower densityxe2x80x9d or xe2x80x9cfluxxe2x80x9d), which can damage tissues in the human eye causing serious conditions such as photokeratitus (xe2x80x9cwelder""s flashxe2x80x9d) and cataracts.
Accordingly, several laser safety standards have come into existence that specify and regulate the parameters of lasers operating in environments that may expose the human eye to laser radiation. In general, three main aspects of regulations exist for lasers and their usage: Class definitions, Accessible Emission Limits (AEL), and Maximum Permissible Exposure (MPE). The class definitions provide non-technical descriptions understandable to lay-persons, AELs define the classification breakpoints, and MPEs are based on biophysical data and indicate actual tissue damage thresholds.
Class definitionsxe2x80x94for example, Class 1, 2, 3, or 4xe2x80x94provide an abbreviated way to readily communicate a hazard level to a user. Class 1 represents lasers that are safe under reasonably foreseeable conditions, including the possibility of a human eye being exposed, either aided (e.g., through binoculars) or unaided, to a laser beam. At the other end of the spectrum, a Class 4 laser is capable of producing hazardous diffuse reflections that may pose skin and fire hazards. As an example, to meet the most stringent standardxe2x80x94class 1xe2x80x94a laser operating at 785 nm must be limited in power density such that the power collected by a human eye exposed to the laser is no greater than 0.56 milliwatts (the class 1 AEL for 785 nm lasers). Various factors such as the distance from the eye to the laser during exposure, and whether the viewing is aided or not, have a significant impact on how much power is collected by the eye.
The present inventors recognized that, while increased demand for WON bandwidth and link range generally would require the power densities of lasers used in WON transceivers to be increased, eye safety standards and concerns for human ocular safety represent strict limits on increasing such power densities. For example, the optical transceiver shown in FIG. 2 uses 622 megabits per second in both directions. However, beyond some levelxe2x80x94for example, 1.2 gigabits per secondxe2x80x94more power would be required to sustain the data rate. Accordingly, the present inventors developed systems and techniques that, among other advantages, enable laser output devices such as WON transceivers to transmit and receive data at increased bandwidths but without exceeding existing safety standards and without increasing risks to humans.
Implementations of the systems and techniques described here may include various combinations of the following features.
In one aspect, an optical transceiver such as used, for example, in a wireless optical network (WON), may include multiple laser sources including a first laser source configured to transmit a first output channel beam having a first optical characteristic and at least a second laser source configured to transmit a second output channel beam having a second optical characteristic; multiple detectors including a first detector configured to detect a first input channel beam having the first optical characteristic and at least a second detector configured to detect a second input channel beam having the second optical characteristic; and multiple apertures including a first aperture through which the first output channel beam and the second input channel beam pass and a second aperture through which the second output channel beam and the first input channel beam pass.
In an embodiment, the first optical characteristic may be a first wavelength (e.g., 830 nm) and the second optical characteristic may be a second wavelength different from the first wavelength (e.g., 785 nm). A difference between the first wavelength and the second wavelength is about 50 nanometers or greater. One or more of the wavelengths may be between 1530 and 1570 nanometers.
In another embodiment, the first optical characteristic may be a first polarization (e.g., transverse electric polarization) and the second optical characteristic may be a second polarization different from the first polarization (e.g., transverse magnetic polarization).
Laser sources that may be used include laser diodes, gas lasers, fiber lasers, and/or diode-pumped solid state (DPSS) lasers. In an embodiment, a laser diode is used that emits an output field that is either substantially transverse electric or substantially transverse magnetic.
Detectors that may be used includes an avalanche photodiode with a bandpass filter or an avalanche diode with a polarizer, for example, a transverse electric polarizer or a transverse magnetic polarizer.
The aperatures may include a lens, for example, a plano aspheric lens having a diameter of about 75 mm.
In an embodiment, the transceiver further may include multiple beamsplitters, including a first beamsplitter associated with the first aperture and a second beamsplitter associated with the second beamsplitter, which differentiate between the first and second optical characteristics. At least one of the beamsplitters may be an optical highpass filter such as a dichroic mirror. At least one of the beamsplitters may be a polarizing beamsplitter. One or more of the beamsplitters may pass beams of the first optical characteristic and reflect beams of the second optical characteristic. Alternatively, or in addition, one or more of the beamsplitters may pass beams of the second optical characteristic and reflect beams of the first optical characteristic.
In an embodiment, the first output channel beam passes through the first beamsplitter to the first aperture, the second input channel beam is reflected by the first beamsplitter to the second detector, the second output channel beam is reflected by the second beamsplitter to the second aperture, and the first input channel beam passes through the second beamsplitter to the first detector.
The transceiver further may include a third laser source configured to transmit a third output channel beam having a third optical characteristic, and a third detector configured to detect a third input channel beam of the third optical characteristic.
The transceiver may include at least two beamsplitters each configured to differentiate between the first and second optical characteristic. In that case, the laser sources, detectors and beamsplitters are arranged relative to each other such that, when the transceiver is operating, the first output channel beam will be passed and the second input channel beam will be reflected by a first beamsplitter, and such that the first input channel beam will be passed and the second output channel beam will be reflected by a second beamsplitter.
In another aspect, an optical transceiver includes a plurality of dichroic mirrors, each of which is configured to pass a beam of a first wavelength and reflect a beam of a second wavelength. The optical transceiver further includes multiple laser sources including a first laser source arranged to transmit a first output channel beam of the first wavelength through a first dichroic mirror and a second laser source arranged to transmit a second output channel beam of the second wavelength that is reflected by a second dichroic mirror. The transceiver also includes multiple photodetectors, including a first photodetector configured to detect a first input channel beam of the second wavelength reflected by the first dichroic mirror and a second photodetector configured to detect a second input channel beam of the first wavelength passed by the second dichroic mirror. The transceiver also includes multiple lenses including a first lens arranged to focus the first output channel beam and the first input channel beam and a second lens arranged to focus the second output channel beam and the second input channel beam.
The first and second lenses may be physically separated (e.g., by about 25 millimeters or greater) to increase eye safety. Further, the physical dimension of the lens (e.g., about 75 mm diameter) may be selected to increase eye safety.
In another aspect, an optical transceiver includes a laser source configured to transmit an output channel beam having a first optical characteristic; a photodetector configured to detect an input channel beam having a second optical characteristic different from the first optical characteristic; an aperture through which the output channel beam and the input channel beam pass; and a beamsplitter, arranged in an optical path of the aperture and the laser source, and configured to pass the output channel beam from the laser source to the aperture and to reflect the input channel beam from the aperture to the photodetector. The first and second optical characteristics may be different wavelengths and/or different polarizations.
In another aspect, performing wireless optical communication may be performed by using a first aperture to transmit a first output channel beam having a first optical characteristic and to receive a first input channel beam of a second optical characteristic different from the first optical characteristic; and using a second aperture to transmit a second output channel beam having the second optical characteristic and to receive a second input channel beam of the first optical characteristic. Further, at least one beamsplitter may be used to differentiate between the first and second optical characteristics, which may be different wavelengths and/or different polarizations. Data may be impressed upon the either or both of the first and second output channel beams using one or more of the following techniques: on/off keying, phase-shift keying, pulse-position modulation, and/or frequency-shift keying.
In another aspect, an optical transceiver may include multiple laser sources including a first laser source configured to transmit a first output channel beam having a first optical characteristic and at least a second laser source configured to transmit a second output channel beam having a second optical characteristic; multiple detectors including a first detector configured to detect a first input channel beam having the first optical characteristic and at least a second detector configured to detect a second input channel beam of the second optical characteristic; and multiple apertures including a first aperture through which the first and second output channel beams pass and a second aperture through which the first and second input channel beams pass.
In another aspect, a wireless optical network may include multiple optical transceivers, each of which is in communication with at least one other optical transceiver. Each of at least two of the optical transceivers may include the following: multiple laser sources including a first laser source configured to transmit a first output channel beam having a first optical characteristic and at least a second laser source configured to transmit a second output channel beam having a second optical characteristic; multiple detectors including a first detector configured to detect a first input channel beam having the first optical characteristic and at least a second detector configured to detect a second input channel beam of the second optical characteristic; and multiple apertures including a first aperture through which the first output channel beam and the second input channel beam pass and a second aperture through which the second output channel beam and the first input channel beam pass.
One or more of the following advantages may be provided. The techniques and methods described here result in an optical transceiver that provides dramatically increased bandwidth compared with a conventional transceiver but without any corresponding increase in ocular safety risks. By separating the output beams, the potential power collected by an observer""s eye can be maintained at safe levels, while at the same time, providing roughly twice or more the total power output for the transceiver as a whole. Enabling the use of different techniques to differentiate the beams (e.g., based on wavelength or polarization) provides design and implementation flexibility.
Moreover, the systems and techniques described here enable the total bandwidth of an optical transceiver to be scalable to a high degree.
The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.